I don't read the blurb on the back of books until after I've read the actual book because it's rare for them not to give the game away to some extent, but Andersen Press has resisted blurting out what lies at the heart of this story.

My job of reviewing When You Reach Me would be almost impossible without giving certain aspects away, though. So if you don't want to know more – for fear of spoiling it – other than that it's a well-written and engaging read (for which the author, Rebecca Stead, has been garlanded with numerous awards in her native US, including the coveted Newbery Medal), then stop HERE. The rest of you, come with me.

Where to start? One day, 12-year-old Miranda comes home to find a message in her apartment. It contains a strange request that she write a letter saying where she has hidden the apartment's spare key. What's puzzling, of course, is that whoever left the message must have used a key to get into the apartment in the first place. This is Miranda's first clue. As time goes on, it becomes apparent that we are in the midst of a time-travel story. The person leaving the notes is from Miranda's future. If she writes him a letter explaining where the key is, then when he travels back into his past/Miranda's present, he can use the key to get into the apartment to leave a letter to ask her where the key is to get into the apartment . . . You get the picture.

Once the truth has dawned, there's the whole matter of who this person from the future might be, and exactly why they're travelling back in time. The writer of the notes proves that he's for real by leaving tantalising clues about Miranda's immediate future, but in a most cryptic and low-key manner. And this is a big part of the book's charm: though time travel is the frame around which the story is constructed, it's really a beautifully observed story about family and friendship: her mother mugging up for her appearance on a quiz show; her mum's boyfriend being near perfect but for one leg being shorter than the other; the laughing man (the local crazy guy); and, of course, her school friends and best friend Sal.

It is also very American. It's useful to know, for example, that – unlike British post boxes – American mail boxes are squat and stand on four feet; that "barrettes" are hair-slides; and that two-dollar bills are much rarer than one-dollar ones. Vital? No, but it does help to show that this book will have a very different feel for a UK reader. It's not just the past which is a foreign country.

One element that jars slightly is Miranda's favourite book, with its time travel element. This works fine as a plot device – giving the characters an opening to discuss such matters – but, despite the book's heroine being named, we're never told its title. From the acknowledgment to Madeleine L'Engle at the end, I take it to be A Wrinkle in Time, but I wish Stead had said so. The book is a favourite with Stead in real life; she somehow treats it differently from the rest of her wonderfully believable world. But this is a minor niggle about a story in which characters really come alive during those few months we spend with them, when their lives are shaped for ever.